The invention relates to aluminum salts of phosphinic acids, processes for their preparation and to their use as flameproofing agents.
Aluminum salts of phosphinic acids are valuable flameproofing agents for polyester and polyamide molding materials. They are prepared from the phosphinic acids in aqueous solution with metal carbonates, metal hydroxides or metal oxides (EP-A2-0699708).
To date, phosphinic esters have been converted into the corresponding phosphinic acids in good yields on hydrolysis at 180xc2x0 C. under pressure with excess water only when the alcohol formed is removed as a mixture with water from the gas phase of the autoclave (Houben-Weyl, Methoden der organischen Chemie [Methods of organic chemistry] 1982, Volume E2, page 142; DE-A1-27 45 982).
It was found that the aluminum salts of phosphinic acids can be prepared in good yields under pressure from the corresponding esters with water and aluminum hydroxide. Furthermore, it was found that the aluminum salts of phosphinic acids or diphosphinic acids having alkyl and/or aryl substitutes are formed in a novel crystal form at a temperature above 150xc2x0 C. under pressure. These aluminum salts are defined as a high-temperature modification. The aluminum salts known to date and prepared at temperatures of 80 to 100xc2x0 C. are defined as a low-temperature modification.
The invention thus relates to aluminum salts of phosphinic acids or diphosphinic acids having alkyl and/or aryl substituents, obtainable by heating an ester of the corresponding phosphinic acids or diphosphinic acids with aluminum hydroxide at a temperature of more 150xc2x0 C. under pressure in the presence of water.
The invention also relates to aluminum salts of phosphinic acids or diphosphinic acids having alkyl and/or aryl substituents, obtainable by heating the corresponding aluminum salts in the low-temperature modification at a temperature of more than 150xc2x0 C. under pressure in the presence of water.
Alkyl substituents are preferably C1-C8-alkyl, in particular C1-C4-alkyl, linear or branched, for example methyl, ethyl, n-propyl, isopropyl, isobutyl, n-butyl, n-hexyl or n-octyl. A preferred aryl substituent is the phenyl group.
The aluminum salts of the high-temperature modification may also be polymers of the phosphinic acid or disphosphinic acids. The terms xe2x80x9cphosphinic acidxe2x80x9d and xe2x80x9cdiphosphinic acidxe2x80x9d, accordingly, include monomers, oligomers and polymers.
The invention furthermore relates to a process for the preparation of aluminum salts of phosphinic acids or diphosphinic acids, esters of the corresponding phosphinic acids or diphosphinic acids being reacted with aluminum hydroxide under pressure at 150 to 350xc2x0 C. in the presence of water.
The invention furthermore relates to a process for the preparation of aluminum salts of phosphinic acids or diphosphinic acids of the high-temperature modification, the corresponding aluminum salts in the low-temperature modification being exposed to a temperature in the range of 150 to 350xc2x0 C. in the presence of water.
Suitable aluminum salts of phosphinic acids or diphosphinic acids are, for example, compounds according to the formula I or II 
in which
R1, R2 are C1-C8-alkyl, preferably C1-C4-alkyl, linear or branched, e.g. methyl, ethyl, n-propyl, isobutyl, n-butyl, n-hexyl, phenyl
R3 is C1-C10-alkylene, linear or branched, e.g. methylene, ethylene, n-propylene, isopropylene, n-butylene, n-decylene,
an arylene, e.g. phenylene, napthylene;
alkylarylene, e.g. methylphenylene, ethylphenylene, methylphenylenemethyl;
arylalkylene, e.g. phenylmethylene, phenylethylene.
The aluminum salts according to the formula I or II can be prepared, for example, from phosphinic esters or diphosphinic esters of the formula III or IV 
in which R1, R2 and R3 have the abovementioned meaning and R4 is C1-C8-alkyl, preferably C4-C6-alkyl. Advantageously, the compounds according to the formula III or IV are reacted with water and aluminum hydroxide under pressure at 150 to 350xc2x0 C. and heated for several hours in this temperature range, the corresponding aluminum salts being obtained as the high-temperature modification.
Particularly suitable starting materials for the preparation of the aluminum salts are: methyl dimethylphosphinate, ethyl ethylmethylphosphinate, isobutyl ethymethyl phosphinate, n-butyl methylpropyl phosphinate, amyl isobutylmethyl phosphinate, isopropyl hexylmethyl phosphinate, n-butyl methyloctyl phosphinate, n-butyl methylphenylphosphinate, n-pentyl diphenylphosphinate, di-n-butyl hexane-1,6-di(methylphosphinate) and diisobutyl benzene-1,4-di(methylphosphinate).
The process for the preparation of the high-temperature modification is carried out in a variant in such a way that phosphinic esters are heated with excess water and stoichiometric amounts of aluminum hydroxide under pressure to 150 to 350xc2x0 C., preferably 180 to 250xc2x0 C., expediently with continuous stirring. The required reaction times are in general at least 4 and preferably at least 40 hours, in particular 50 to 70 hours. The reaction times are dependent on the chain length of the ester group (for example, R4 in the formulae III and IV). Methyl esters hydrolyze, for example, more rapidly than the butyl esters. After the end of the reaction, the resulting crystals are isolated and dried. The reaction conditions, such as temperature and reaction time, can also be varied so that mixtures of low-temperature modification and high-temperature modification are obtained.
Aluminum salts of the high-temperature modification are gradually transformed at room temperature into the low-temperature modification, but the crystal habit of the high-temperature modification is retained.
The high-temperature modification consists of acicular crystals which have a fibrous appearance. The crystals generally have a width in the range from about 0.2 to 1.5 xcexcm, in particular 0.2 to 1 xcexcm. The ratio of width to length of the crystal has, as a rule, a value of less than 0.2. The ratio of width to length of the crystal is usually about 0.1 or less. The crystals usually form agglomerates which have the appearance of felt-like bales. The agglomerates are obtained with a diameter in the size range from about 10 to 170 xcexcm, the main proportion comprising agglomerates having a diameter of about 20 xcexcm.
The low-temperature modification consists of cylindrical crystals which as a rule have a width of more than 1.5 xcexcm, typically 2 to 5 xcexcm. The ratio of width to length of the crystals is usually in the range from 0.6 to 2, typically 2. The crystals also form agglomerates which for the most part have a diameter of about 20 xcexcm.
The high-temperature modification of the aluminum salts is particularly suitable as flameproofing agents or flame retardants for plastics, in particular for polyester or polyamide.
The aluminum salts of the high-temperature modification have advantages for use as flameproofing agents for plastics. Investigations have shown that the flame-retardant effect of the salts of the phosphinic acids in the plastic depends on the particle size of the salt and that an optimum particle size results. The aluminum salts of the high-temperature modification form directly in the advantageous particle size. Milling of the aluminum salts is therefore not necessary for compounding, which means an economic advantage for the preparation of flameproofed plastics. In addition, the finer structure of the aluminum salt permits a more homogeneous distribution of the flameproofing agent in the plastic. The fibrous structure of the aluminum salt moreover leads to reinforcement of the plastic (fiber reinforcement). The finer structure leads to a larger surface area of the particle and, for example with the use of synergistic agents, such as nitrogen compounds, improves their flameproofing effect.